Methods and apparatus for air classifying and screening of finely divided material



March 25, 1969 D suL v 3,434,593

METHODS AND AFPARATUS FOR AIR CLASSI FYING AND SCREENING OF FINELYDIVIDED MATERIAL Filed Jul ye, 1966 Sheet of s 1 i l l P L M E He.a .L;I... 6 f2 an a 3 & A46

M I... 6a a I ea March 25, 1969 J, SULUVAN 3,434,593

METHODS AND APPARATUS FOR AIR CLASSIFYING AND SCREENING OF FINELYDIVIDED MATERIAL Filed July 6. 1966 Sheet 2 of'3 E a 6 64 L 64 1e I6 I,

March 25, 1969 [3.1 SULLIVAN 3,434,593

METHODS AND APPARATUS FOR AIR CLASSIFYING AND SCREENING 0F FINELYDIVIDED MATERIAL Filed July 1966 Sheet 5 of a United States Patent 0 f3,434,593 METHODS AND APPARATUS FOR AIR CLASSIFY- ING AND SCREENING OFFINELY DIVIDED MATERIAL Daniel J. Sullivan, Milton, Mass., assignor toSturtevant Mill Company, Boston, Mass., a corporation of MassachusettsFiled July 6, 1966, Ser. No. 565,648 Int. Cl. B07b 9/02 US. Cl. 209-3114 Claims ABSTRACT OF THE DISCLOSURE An air classifier for classifyingfinely divided materials comprises inner and outer casing members and afan member for inducing a flow of air upwardly through cascadingparticles introduced at the top of the casing members. Adjustable screenmeans of the invention are located adjacent to the fan member andfunction to selectively provide for a separation of coarse particles notremoved by conventional air classifying apparatus.

This invention relates to improved methods and ap paratus for processingfinely divided materials and, more particularly, the invention isconcerned with machines for air classifying and screening finely dividedsubstances such as minerals, vegetable substances, chemicals and othersimilar products. V

In Patent No. 2,774,476, owned by the assignee of the present invention,a typical form of air classifying ap paratus of the general typereferred to is disclosed wherein a cascading mass of finely dividedmaterial is subjected to a centrifuging action and the centrifugedmaterial is displaced radially outwardly into a fan-induced current ofair flowing upwardly through the centrifuged mass to obtain a separationof fine particles from relatively coarser particles.

Machines of the class referred to are very effective in removingcontrolled percentages of relatively fine particles and separating themfrom relatively coarse particles. This is accomplished by the fineparticles being lifted upwardly by a fan-induced flow of air and thesefine particles are led into an outer casing which is separated from aninner tailings casing for coarse particles.

It will be appreciated that there are limitations in the capability ofsuch machines to lift and separate some ranges of relatively largersized particles. More recently it has been found that industrys need forspecialized classifications of finely divided materials have createdrequirements for extending the range of classification on the coarseside. One such instance is exemplified in requirements for processing aproduct such as limestone, and particularly in the preparation of anagricultural limestone where definite percentages of slightly coarserparticles, together with controlled percentages of fines, are now beingspecified by users.

It is a chief object of the present invention, therefore, to improve airclassifying machines of the class referred to and, in particular, todevise an improved method and means for carrying out air classifyingoperations and simultaneously extending the range of particle sizesproduced on the coarse side for meeting requirements of users ofagricultural limestone and similar products.

3,434,593 Patented Mar. 25, 1969 Another object of the invention is toprovide a method of combining the steps of air classifying with novelmechanical screening in such a manner that there may be realized acontrolled extension of the range of coarse particle sizes recoveredwhile also obtaining a desired recovery of relatively fine particles ina suitable product mixture.

Another object of the invention is to provide an improved apparatus forprocessing finely divided material such as limestone and the likewhereby standard forms of air classifying machines of the type disclosedin the above-noted patent may, by relatively simple modification, beadapted to a novel type of screening operation.

Still another object is a method of combined classifying and screeningof finely divided materials to produce a product mix occurring in arange of predetermined relatively coarse sizes from which a specifiedrange of fines may be substantially removed.

The nature of the invention and its objects will be more fullyunderstood and appreciated from the following description of a preferredembodiment of the invention selected for purposes of illustration andshown in the accompanying drawings, in which:

FIGURE 1 is a plan view of an air classifying apparatus of the inventionin one preferred form;

FIGURE 2 is a side elevational view with portions broken away at oneside thereof;

FIGURE 3 is a vertical cross section of the machine shown in FIGURES 1and 2;

FIGURE 4 is another fragmentary cross sectional view of the machine withportions thereof broken away to indicate diagrammatically the path offlow of fine particles and coarse particles induced by rotative movementof a fan member; and

FIGURE 5 is a detail fragmentary exploded view of a portion of the innercasing of the machine and screening means of the invention.

With the foregoing objectives in mind, I have discovered that inutilizing a machine of the general type shown in the drawings for airclassification of fines particles, it is possible to carry out, Withoutinterference with normal air classification, a unique and importantmechanical screening step. This mechanical screening step can be veryprecisely regulated and controlled to not only isolate a specific rangeof coarse particles, but also to introduce these particles into thedischarge flow of air classified fines while allowing relatively coarserparticles to pass downwardly through a tailings conduit in the usualmanner.

I have found that I may accomplish this screening operation bydisplacing particles of a specific desired size radially outwardlythrough protectively arranged screen means having openings orperforations arranged to avoid interference with normal air classifyingand whose size, arrangement, and accessibility in the machine may beadjusted in accordance with a particular coarse particle sizerequirement.

In one preferred embodiment of the invention, I provide a plurality ofscreen elements detachably secured in the walls of an inner casing ofthe air classifier and provided with adjustable screen covers by meansof which the screen apertures may be opened or closed as desired toregulate passage of particles outwardly therethrough.

The screen elements are disposed immediately below radially projectingcasing top edges and protective flange portions so that the screenelements occur in a relatively sheltered relationship and fine particlespassing downwardly over the outer edges of the inner casing top are thusprevented from entering between the screens and the screen covers. Inthis preferred embodiment I further locate the screen elements in theinner casing of the machine in positions so chosen with respect to theair classifier fan as to subject particles to a maximum exposure of theradially displaced stream of air occurring in the immediate vicinity ofthe fan tips.

Considering first the conventional portions of the air classifyingstructure in the drawings, numeral 2 denotes a closed cylindrical casingwhich is formed with a conically shaped lower section 4 and a topsection 2a. The section 2a has vertically supported therethrough abearing B in Which is received a shaft S driven by a belt L and a motorM. The bearing B and the motor M are preferably supported on transversebeam members 3 in turn secured at the upper side of the top section 2a.A pulley P is driven by the belt L as suggested in FIGURE 3. Located inspaced relation to the outer casing 2 is an inner cylindrical casing 30carried on supporting brackets 32 projecting inwardly from the innerperiphery of casing 2. The casing 30 is provided with a top rim portion31 which projects radially outwardly as shown in FIGURES 3 and 4 for anappreciable distance and also provided immediately under the rim 31 arereinforced channel shaped members 33 providing overhanging lower edges35.

At the lower end of shaft S is provided a rotary a'pparatus includingthe distributor plate 10 which is supported on suitable bearing means10a. Located in spaced relation above the distributor plate 10 androtatable with it is a plate member 11 to which is secured vanes as .13.The spacing of the vanes 13 may be varied to provide a means ofrejecting certain ranges of coarse particles as is disclosed in PatentNo. 2,774,476.

Finely divided material is delivered into the machine through an inletchute 6 formed at the upper side of the machine as shown in FIGURE 3.Also supported for rotation with the distributor plate 10 and the member11 is a fan member F which is adapted to rotate and induce an upwardlydirected pulling force on material centrifugally displaced from thedistributor plate 10.

It is pointed out that the inner casing 30 extends upwardly to a pointjust below the fan member F as shown in FIGURE 3 and also includes alower conically shaped section 12 around which is supported a pluralityof adjustable vanes. Supported immediately below the vanes is a secondconically shaped section 15, at the lower extremity of which is attacheda tubular tailings conduits 18. The latter member projects downwardlythrough one side of the conical section 4 as best shown in FIGURE 3. Aswill be observed in FIGURE 3 the tailings conduit 18 occurs in spacedrelation to inner peripheral portions of the conical section 4 so as todefine a fines outlet 24.

In the conventional operation of the parts now described, finely dividedmaterial is directed into the feed inlet 6 and passes downwardly aroundthe bearing and rotating shaft S to fall onto the rotating plate 10. Asthe material drops down onto the distributor plate it is centrifugallydisplaced towards the inner casing 30, and some of the relatively coarseparticles begin to fall downwardly around the inner peripheral surfacesof the conical section12.

As particles start to fall the upwardly induced draft forces of the fanF, acting upwardly through the path of movement of the particles, liftssome of the falling fines and travels these fines upwardly and inwardlythrough the rotating vanes 13 and the fan F and then outwardly anddownwardly through the passageway 17 between the outer casing 2 and theinner casing section 30.

From passageway 17 the fines drop downwardly through the conical section4 to be discharged from the fines conduit outlet 24. The remainingcoarse particles of the finely divided material pass downwardly throughthe conical 4 section 12 and then out through the conical tailingsconduit 15 and discharge outlet 18.

In accordance with the method of the present invention, relatively fineparticles are classified as described above and, in addition, I providefor discharging a selected range of relatively coarse particles radiallyoutwardly through the inner casing section 30 and into the path ofdownward fiow of the fines particles passing down through passageway 17.

As one suitable means of accomplishing this I construct the inner casingmember 30 with a plurality of spaced apart openings 40, one of which isshown in FIGURES 2 and 5. The openings 40 may, if desired, be fittedwith enclosing frames of some suitable nature detachably securedtherein. In these framed openings 40, I further provide a plurality ofscreen elements as 42 (FIGURE 2). The screen elements may, for example,consist of rectangularly shaped plates of perforated metal or othersuitable screen material and each of the screen elements are furthercurved into an arcuate shape to coincide with the curvature of thecylindrical casing 30.

In the arrangement of screen elements 42, shown in the drawings, it willbe observed that the upper edges of the screen elements are locatedclose to and directly under the overhanging portions of edges 35 of thereinforcing members 33, and it is also pointed out that the outer edgesof the casing rim 31 extend somewhat further beyond the reinforcingmembers 33 so that the screen elements are thereby disposed in asomewhat recessed and sheltered position. Therefore, fine particlesmoving downwardly over the rim 31 and through the passageway 17, areprevented from being drawn inwardly towards the screen elements.

Cooperating with the screen elements and arranged to move against andclose the openings in the screen elements are complementary curved coverelements 44 which as shown in FIGURES 2, 3 and 4 are supported on rods46 slidably received in bars 48 in the outer casing 2. By means of thisarrangement the screen elements may be opened or closed as desired by amanual adjustment of any one or all of the rods from outside themachine. It will be understood that any desired number of screens andscreen covers may be employed spaced around the casing members assuggested in FIGURE 1.

An important feature of the invention is the location of the screenelements and screen covers in the recessed position described relativeto the casing rim 31 to provide a sheltering action. It will beappreciated that when the fan F is revolving at relatively highoperating speeds, it exerts a considerable upward suction force in thedirection indicated by the arrows A and there conceivably could arise atendency for this suction force to draw fine particles from thepassageway 17 inwardly through the screen elements unless this isprevented. This would result in an undesirable recycling of fineparticles.

I find that by maintaining the screen elements in a recessed positionunder the casing rim 31 and the flanged portions 35, particles fallingdownwardly through the passageway 17 are effectively prevented fromcoming into contact with the screen elements. Also, by varying thespaced relation of the screen covers 44 to respective screen elements, Ifind that a partial vacuum effect may be induced in the spaces betweenthe screen elements and the screen covers so that no fines are drawn inand little resistance is presented for centrifuging particles to bedirected against the screen element peripheries and out through theseopenings. Therefore, a selected range of sizes may be discharged throughthe screen element openings.

It will be evident that I may vary the size of the openings in thescreen elements to control the range of particle sizes which arescreened. It will also be apparent that by closing some of the screenelements and leaving others open, greater or smaller quantities of anydesired range of screened products may be realized. It is still furtherpointed out that these operations can be carried out withoutinterference with the normal air classifying of fines in the usualmanner. The result is a more carefully regulated product mix of desiredfines and a controlled percentages of coarse particles withinpredetermined limits.

In the preferred arrangement referred to, the screen elements 32 aredisposed so as to form a multiplicity of openings all the way around theinner peripheral surface of the casing 30. Moreover, the tops of thesescreen elements are located immediately below the tips of the fan F. Ifind that in this region of close proximity to the tips of fan F,centrifuged material is continuously swept around and against the screenopenings by the air stream of the fan and a maximum screening action isproduced as regulated by the size of the openings employed in the screenand the speed of the fan induced.

In operation material is fed into the member 6 and passes downwardlyonto plate 10. The combined effect of the fan F, the rotating vanes 13,and distributor plate 10, acting on the continuously cascading mass ofparticles, is to provide a spiralling stream of air and particles ofvarying sizes which move outwardly in response to centrifugal forces.

As indicated diagrammatically in FIGURE 3, some of the relatively finerparticles in response to the pulling forces of the fan F move outwardlyonly a short distance and then are lifted and travelled inwardly andupwardly as indicated by the arrows A. Other relatively coarse particlesare centrifuged outwardly a further distance and start to move in adownwardly spiralling stream along the inner coned surface of the casing30 as suggested diagrammatically by the arrows D.

In accordance with the invention still other relatively coarse particlesof a selected range of sizes are displaced radially outwardlysufficiently to move against and pass through apertures in the screenelements 42 and then into the passageway 17 as denoted by the arrows T.These selected coarser particles are thus combined with the airclassified fine particles passing down through passageway 17 and aproduct mixture of air classified fines and a predetermined percentageof screened coarse particles is thus obtained.

As illustrative of one typical application of the method of theinvention, there may be cited processing of limestone aggregates forproducing agricultural limestone with a required percentage of bothfines and coarse particles regulated to meet specific customerrequirements.

It should be understood that in conventional processing of limestoneaggregates to produce agricultural limestone, standard requirements havein the past specified that particles be of a size such that a 1000 poundsample of material be sutficiently finely divided to pass through a 16mesh screen and 95% of the sample should pass through a .30 mesh screen.However, more recent requirements in the trade now specify that a 1000pound sample of agricultural limestone include a fraction of at least20% of particles of a size sufliciently coarse to be retained on a meshscreen with 80% of the material passing through a 10 mesh screen.

Since a 10 mesh screen has openings of approximately .062 inch and a 100mesh screen has openings of approximately .0058 inch, the requirement isquite selective and involves retaining coarse particles which cannot besatisfactorily classified by conventional air classifying machines. Thatis so since the large size particles are not effectively lifted by afan-induced air stream. By using the method and apparatus of theinvention however, as described above, the requirements can be metsatisfactorily and I have indicated below two examples which compareconventional air classifying operations with combined air classifyingand screening of this invention.

Example I A conventional agricultural limestone product was producedfrom 1000 pounds of limestone aggregates having the following analysis:

650 pounds of particles passed through. a 14 mesh screen and wereretained on a 10 mesh screen,

150 pounds of particles passed through a 10 mesh screen and wereretained on a mesh screen,

200 pounds of particles passed through a 100 mesh screen.

This example, after being subjected to an air classifying operation ofconventional nature, provided 20 pounds of particles which were held ona 100 mesh screen, and 1160 pounds of particles which passed through a100 mesh screen.

From this analysis of the sample processed, it will be observed that theconventional air classifier failed to provide the required percentage ofcoarse particles and the processed fines fraction, together with thecoarses removed, held only 180 pounds of processed agriculturallimestone.

Example II There was provided a similar 1000 pound sample of limestoneaggregates with the same relative percentages of coarses and finesspecified in the sample of Example I, i.e.

650 pounds were retained on a 10 mesh screen,

150 pounds passed through a 10 mesh screen but were retained on a 100mesh screen,

200 pounds passed through a 100 mesh screen.

This sample was subjected to the combined air classifying screeningsteps of the invention as regulated by the use of screen elements 42.The screen elements for this particular operation were formed withopenings of approximately inches in size in order to pass particleswhich would be held on a 10 mesh screen (.065 inch).

The resulting product from the combined air classifying and screeningsteps produced the following:

200 pounds of particles held on a 10 mesh screen,

pounds passed through a 10 mesh screen and held on a 100 mesh screen,

200 pounds passed through a 100 mesh screen.

It is pointed out that this product had the required quantity of coarseparticles 20% (200 pounds) as specified above and held a total productmix of 540 pounds of acceptable agricultural limestone.

It will be understood that the openings of screens 42 may be varied asdesired to meet any given specification. It may also be desired toincrease the number of screen elements used by forming openings at otherpoints in the cylindrical casing members and locating screen elementstherein at different levels. For example, I may as shown in FIGURES 2and 3, form openings in the conical lower section 12, and I may mounttherein screened elements as 60 which are secured to rods as 62 slidablysupported in bearings 64 in the outer casing 2.

These relatively lower screen elements may, I find, be designed to passcertain sizes of coarse particles which are not readily processed atother points in the casing because of their rapid downward path oftravel. Similarly, there may be employed at even lower levels stillother screen elements as, for example, screen element 66 fixed to rods68 slidably disposed in bars in the lower conical section 4.

The invention may also be practiced utilizing other forms of screenmembers such as screen cloth, spaced bar sections, and the like. Also,the invention may be utilized with a wide range of finely dividedmaterials other than agricultural limestone and various other advantagesmay be realized. For example, in making concrete sand, it may berequired to have a specified range of coarse particles and only alimited percentage of fines of some smaller range of sizes, and byutilizing the combined air and screen operation there can be realized asharper control the quantity of fines present in the coarse particleswhich are passed through the tailings outlet of the machine and thus abetter concrete sand product is realized for some requirements.

'While I have shown and described preferred embodiments of theinvention, various other changes and modifications may be resorted towithin the scope of the appended claims.

I claim:

1. In a method of air classifying and screening a finely dividedmaterial, the steps which include delivering a free falling mass offinely divided solid material downwardly upon a rotating distributorplate for centrifugal dispersion within a closed chamber having a topportion with an outlet port formed therein and screen means in thesidewalls thereof, supplying air through a conduit means opening intosaid closed chamber below said dispersing mass, evacuating the airsupplied to said chamber upwardly through said finely divided dispersedmaterial by a vacuum fan rotating at a predetermined speed and locatedat the outer side of the closed chamber with its inlet connected withsaid outlet port for entraining relatively fine particles of thedispersed material, travelling the air and entrained fine particlesthrough a rotating sizing device located between the distributor plateand the outlet port of said chamber, discharging the flow of air andentrained fine particles from said vacuum fan into a second closedchamber for deposit of portions of the dispersed material at the bottomportion thereof, and simultaneously displacing air and relativelycoarser particles radially outwardly through the screen means to thesaid second closed chamber and into the path of flow of the fineparticles.

2. A method according to claim 1 and including the further steps ofvarying the screen apertures to regulate the magnitude of the relativelycoarse particles which are passed through the screen member.

3. A method according to claim 1 and including the further steps ofselectively opening and closing the screen apertures to control thequantity of relatively coarse particles which are mixed with the fineparticles.

4. A method according to claim 1 and including the further steps ofinducing a vacuum effect externally of the screen means to opposesuction of relatively fine particles inwardly therethrough.

5. A method according to claim 1 including the further steps of radiallydisplacing coarse particles through screen apertures occurring atprogressively lower levels in the closed chamber.

6. A method according to claim 1 in which the finely divided materialconsists of limestone aggregates occurring as a mixture of coarse andfine particles, said method including the further step of displacingrelatively coarse particles through the screen means in a controlledmanner to constitute at least twenty percent of the total quantity ofair classified particles and screen particles processed to provideagricultural limestone.

7. In an apparatus for air classifying and screening finely dividedmaterial, the combination of an outer closed casing having a cone with adischarge opening at the bottom thereof and a drum above said cone, saiddrum including a top wall portion, an inner closed casing composed of acone with a lower discharge conduit portion extending through the coneof the outer conical casing, said inner closed casing having a drumabove the cone and said drum including a top wall portion with aprojecting rim, said inner closed casing being spaced from the outercasing and defining a chamber therebetween, driven shaft meansprojecting through the top Wall of the outer casing and within the innercasing, a centrifugal distributor plate mounted on the lower end portionof said shaft and spaced from the top wall portion of the drum of saidinner casing, a rotary sizing device positioned within the inner casingabove the distributor plate, means for connecting the shaft, said rotarysizing device and said distributor plate to provide a uniform continuousrotation of the parts, means for introducing finely divided solidmaterial into said inner casing and upon said distributor plate forcentrifuged dispersion, an outlet port formed in the top wall portion ofthe drum of the inner casing, a vacuum fan mounted between the top wallportion of the inner drum and the top wall portion of the outer drum androtatable with said driven shaft for travelling centrifuged materialupwardly through the fan and then downwardly through said chamberbetween the inner and outer casing, said inner drum having a pluralityof screen elements mounted therearound at points immediately below thetips of the said vacuum fan and in recessed relation to the rim of thetop portion of the inner casing, said screen elements being of a sizesutficiently large to provide for passage of centrifuged particlestherethrough and into the said chamber.

8. A method according to claim 7 in which the screen means includes aplurality of adjustable closure plates for opening and closing theopenings in the screen means.

9. In an apparatus for air classifying and simultaneously screening:finely divided material, the combination of an outer casing having abottom outlet for discharging relatively fine particles and an innercasing located in spaced relation to the outer casing to define anannular chamber which communicates with the said bottom outlet, saidinner casing being formed with bottom conduit means located through theouter casing for discharging relatively coarse particles, driven shaftmeans vertically disposed through the outer casing and extendingdownwardly into the inner casing, a distributor plate fixed to theshaft, means for introducing finely divided material into the innercasing onto the rotating distributor plate to provide a centrifugeddispersion of particles in the inner casing, suction fan means mountedin the top of the inner casing for lifting centrifuged particlesupwardly and travelling the particles into the said annular chamber,screen means located adjacent said suction fan means in the wall of saidinner casing, and having openings for selectively passing therethroughrelatively coarse centrifuged particles into said annular chamber.

10. A structure according to claim 9 in which the screen means includesperforated screen plates detachably secured in the inner casing andcover elements slidably disposed in the outer casing for opening andclosing the openings in the said screen plates.

11. A structure according to claim 10 in which the screen plates arearranged in recessed relation to a top rim of the inner casing and thesaid cover means when moved into a position immediately below the saidtop rim cooperates with the screen elements to provide regions ofpartial vacuum for rejecting fine particles in the said annular chamber.

12. A structure according to claim 11 in which the screen elements aredisposed in the inner casing in immediate proximity to the region of thetips of the said fan whereby a maximum eifect of the air stream inducedby the fan is exerted on centrifuged particles to sweep them around theinner periphery of the inner casing and discharge them through theapertures in the screen elements into the annular chamber.

13. A structure according to claim 9 in which the screen means includesan upper row of screen elements arranged around the peripheral surfaceof the inner casing and a second row of screen elements also arranged inspaced relation around the peripheral portions of the inner casing at alower level.

14. A structure according to claim 9 in which the screen means includesa top row of screen elements for discharging a selected range ofrelatively coarse particles, a second row of screen elements below thefirst row for discharging another range of coarse particles and a thirdrow of screen elements below the second row for discharging stillanother range of coarse particles.

(References on foliowing page) References Cited UNITED STATES PATENTSFOREIGN PATENTS 694,739 8/ 1940 Germany.

Anderson 209--305 X Fraser 209 148 X TIM R. MILES, Przmary Exammer.Sturtevant 20937 X 5 L Sturtevant.

Doyle

